The authors study data pollution attacks in wireless inter-flow network coding systems. Although several defenses for these attacks are known for intra-flow network coding systems, none of them are applicable to inter-flow coding systems. They formulate a model for inter-flow network coding that encompasses all the existing systems, and use it to analyze the impact of pollution attacks. Their analysis shows that the effects of pollution attacks depend not only on the network topology, but also on the location and strategy of the attacker nodes. They propose Code-Guard, a reactive attestation-based defense mechanism that uses efficient bit-level trace-back and a novel cross-examination technique to unequivocally identify attacker nodes.